This invention relates to channel decoders.
In many of today's advanced communication systems, channel coding is a key ingredient. The transmitter partition's the data stream into blocks of bits (packets) that are encoded to introduce redundancy information into the transmitted block. The encoded data block is modulated and transmitted through the communication link (channel) connecting the transmitter to the receiver, and a channel-corrupted version of the transmitted data block is received at the receiver-end. After down-conversion and demodulation, the channel decoder at the receiver uses the redundancy introduced by the encoder to recover the transmitted information more reliably.
In general, channel decoders are often categorized by the combination of input/output values they accept or provide. For example, a hard-input hard-output (HIHO) decoder accepts a stream of bits (binary values) as input and provides a stream of binary bits at its output that represent the estimated transmitted bit sequence. Soft-input hard-output (SIHO) decoders accept as input real-valued levels called soft symbols or soft bits that represent the reliability of the bit value. A SIHO decoder produces a stream of binary bits at its output that represents the estimated transmitted bit sequence.
Turbo codes are a special case of channel coding that can operate very close to the theoretical limits of channel capacity and, therefore, are close to optimal. See, for example, C. Berrou et al., Near Optimum Error Correcting Coding and Decoding: Turbo Codes, 44 IEEE Transaction on Communications 1261 (1996), which addresses parallel concatenated turbo codes and their associated encoders and decoders. Serial concatenated turbo codes are addressed, for example, in S. Benedetto et al., “Serial Concatenation of Interleaved Codes: Performance Analysis, Design and Iterative Decoding,” IEEE Transaction on Information Theory, vol. 44, No. 3, pp. 909–926, May 1998.
Turbo codes typically use another type of decoder, known as soft-input soft-output (SISO) decoder, that not only accepts soft inputs, but also provides soft-output. Thus, in SISO decoders, the reliability of the estimated bits, as well as the estimated bits, is provided. Some SISO decoders use the Bahl, Cocke, Jeinek and Raviv (BCJR) algorithm or the soft-output Viterbi algorithm (SOVA). See L. R. Bahl et al., Optimal Decoding of Linear Codes for Minimizing Symbol Error Rate, IT-20 IEEE Trans. Inform. Theory 248 (1974); G. D. Forney, The Viterbi Algorithm, 61 Proc. IEEE 268 (1973).